Since a polyamide resin has excellent characteristics such as mechanical characteristics as well as resistance to chemicals and moldability, it has been widely utilized in various parts such as automobile parts, electric and electronic parts and industrial machine parts. Although a polyamide resin is relatively excellent in thermal aging resistance, deterioration by the action of heat and light is unavoidable. As to a method for enhancing thermal aging resistance of a polyamide resin, methods wherein copper halide, potassium halide, oxazole compound or the like is added as a thermostabilizer have been known from old time already (for example, Patent Document 1).
As a result of the art as such, a polyamide resin is used in the parts for the use being exposed to high-temperature environments of about 140° C. in the fields of automobile parts and electric and electronic parts.
However, with regard to engine room of automobiles for example, environmental temperature in the engine room has become high as a result of increase in engine output, tendency of arranging the parts with high density, etc. in recent years and there has been a demand for high level of thermal aging resistance which has not been aimed until now.
In order to fulfill this demand, there have been proposed a method wherein polyamide is compounded with finely granulated element iron (Patent Document 2), a method wherein polyamide is compounded with finely granulated and dispersed metal powder (Patent Document 3), a method wherein a mixture of two kinds of polyamides having different melting points from each other is compounded with a copper compound and iron oxide (Patent Document 4), a method wherein polyamide is compounded with a thermostabilizer such as copper iodide or potassium iodide and a composite oxide such as tri-iron tetraoxide (containing iron (II) oxide) (Patent Document 5), etc. It has been said that the products obtained by these methods are excellent in thermal aging resistance even under the environment of as high as about 200° C.
However, in the methods of Patent Documents 2 and 3, there is a danger of ignition during preparation of a composition whereby the preparation is not easy. In the method of Patent Document 4, there is a disadvantage that the effect is achieved only by a very limited composition. In the method of Patent Document 5, there may be the case wherein stability and reproducibility of the thermal aging resistance and the mechanical strength are inferior. As such, any of them is to be still improved.